There are two approaches to providing acid to a phosphoric acid fuel cell to replenish acid loss with time due to evaporation into the reactant streams. There are known acid addition approaches where acid in a liquid or vapor form is continuously or periodically added to the cells. These approaches are complicated and expensive. The more preferred approach is the passive approach where sufficient acid to satisfy the life requirements is incorporated into porous components in the cell during the initial assembly of the cell.
There are many different phosphoric acid fuel cell designs. U.S. Pat. No. 4,374,906 shows a cell with ribbed anode and cathode substrates. The '906 cell has a high acid inventory and good electrolyte management characteristics; but is expensive to manufacture due to the high content of expensive carbon fibers and the high volume of carbon-carbon composite material that must be heat treated. U.S. Pat. No. 5,558,955 shows a cell with a ribbed electrolyte reservoir plate as the anode flow field and a nonporous hydrophobic cathode flow field. The '955 cell also has a high acid inventory and good electrolyte management characteristics; but is still expensive to manufacture due to the high volume of carbon-carbon composite material that must be heat treated. Graphite flow fields are not acceptable because they absorb a significant quantity of acid and thus reduce the life of the fuel cell.
WO/2006/071232 discloses a cell which has nonporous anode and cathode flow fields and all of the acid must be stored within the hydrophilic electrode substrates. This cell represents a lower cost design because the carbon fiber content is minimized and the volume of material that must be heat-treated is substantially reduced. Furthermore, porosity need not be a factor, since the flow fields are both non-porous. The '232 cell has poorer electrolyte management characteristics which are the subject hereof.
Proton-conducting liquid electrolytes which may be used as alternatives to phosphoric acid are known. U.S. Pat. No. 5,344,722 discloses an electrolyte which is a mixture of phosphoric acid and a fluorinated compound or a mixture of phosphoric acid and siloxanes. U.S. Publication No. 2006/0027789 discloses a proton-conducting liquid electrolyte where the anion is a fluoroborate or fluoroheteroborate.
Conventional phosphoric acid fuel cell power plants typically comprise stacks 7 of fuel cells 8, as shown in FIG. 1, the temperature of the fuel cells being controlled by a coolant that passes through cooler plates 9 interposed between groups of between five and ten fuel cells. Referring to FIG. 2, each fuel cell 8 comprises an acid retaining matrix 11 having anode catalysts 12 on one side and cathode catalysts 13 on the other side. The catalysts are respectively supported by a porous anode substrate 16 and a porous cathode substrate 17. Porous anode substrate 16 and porous cathode substrate 17 are hydrophilic as is known in the art. The fuel cells (except at the ends or adjacent to cooler plates) share non-porous, hydrophobic separator plate assemblies 19 which include fuel channels 20 adjacent the anode substrate 16 and air (or other oxidant) channels 21 adjacent the cathode substrate 17. The reactant gases in the channels 20, 21 diffuse through respective substrates 16, 17; hence the reference to gas diffusion layers (GDLs). Adjacent a cooler plate 9, the fuel flow channels 20 may be formed in a fuel flow field plate 23 which does not have air flow channels therein; similarly for the cathode side.
The terms “non-porous” and “hydrophobic”, as used herein with respect to the separator plates 19, mean that the separator plates 19 are sufficiently non-porous and hydrophobic so that substantially no liquid electrolyte penetrates the separator plates.
As shown in FIG. 2, the traditional phosphoric acid fuel cell has a substrate 16 adjacent the anode catalyst 12 which is of substantially the same thickness as the substrate 17 adjacent the cathode catalyst 13.
The acid that is required for operation of the fuel cell, which must sustain the fuel cell for the life thereof, is ideally sealed within the fuel cell at the time of manufacture. The initial acid fill at manufacture is about 35% to 45% of the void volume in both substrates. The life of a phosphoric acid fuel cell is dependent upon the acid being sufficient so that the matrix and seals in substrates are essentially full during operation and is therefore dependent on retaining acid. If the separator plate assemblies 19 are non-porous and hydrophobic, the phosphoric acid required for the life of the fuel cell must be stored within the porous anode and cathode substrates. Because the substrates 16, 17 represent the principal thermal resistance between the coolers 9, substrates as thin as possible are desired to minimize the temperature rise within the cells between the coolers, and thereby maximize the power density of the fuel cell stack as well as to reduce cost.
Any situation which causes the amount of phosphoric acid to exceed the void volume within the porous substrates 16, 17 results in expulsion of the acid from the cell. That is, the acid will necessarily flow into either or both of the reactant flow field channels 20, 21 and eventually into the manifolds and other reactant gas plumbing. This reduces the amount of acid available, thereby fore-shortening the life of the fuel cell, as well as damaging components which may reduce performance and/or further shorten the life of the cell.
As is known, undiluted phosphoric acid (that is, 100 wt % phosphoric acid) freezes or crystallizes at temperatures as high as 40° C. about (100° F.). Crystallizing or freezing of the electrolyte causes a decay in performance of the fuel cell stack. On the other hand, when diluted to on the order of 70 wt %, the acid will remain a liquid to about −30° C. (about −20° F.). Therefore, when shipping newly manufactured phosphoric acid fuel cells, they are conditioned by diluting the phosphoric acid from close to 100 wt % to about 70 wt % to prevent freezing of the electrolyte during shipment.